The present invention broadly relates to devices for holding or supporting semiconductor wafers during fabrication processes, and deals more particularly with a device for supporting a wafer in a rapid thermal processing system.
Various types of devices have been developed in the past to support individual semiconductor wafers within a chamber used to process the wafer as part of a manufacturing operation. Such chambers are used to conduct various types of processing operations under a controlled environment, including chemical vapor deposition (CVD) and physical vapor deposition (PVD). In order to control the temperature within the processing chamber, various types of heating systems can be integrated into the semiconductor processing station, of which the chamber forms a part. One type of heating system used in processing stations is referred to as a rapid thermal processing (RTP) system which is employed to conduct rapid thermal oxidation and rapid thermal annealing of a wafer within the processing chamber. RTP systems have several advantages over conventional furnace systems. For example, one limitation of a furnace system employed to carry out oxidation processes is its inertia to temperature transition, resulting in a higher thermal budget. The thermal budget can be reduced considerably by reducing the duration of these transitions through the use of RTP. In the case of annealing processes wherein annealing removes defects introduced by an ion implantation, the use of RTP systems provides a higher level of dopant activation and annealing effect, compared to conventional furnace systems.
In an RTP system, a semiconductor wafer is rapidly heated from a low temperature to a high processing temperature. It is held at this elevated temperature for a short time and then brought back rapidly to a low temperature. RTP durations at high processing temperatures vary from 1 to 5 minutes. During the RTP process, the wafer is held on a support which in turn rotates the wafer in order to achieve uniform wafer heating. The heat source typically comprises a multiplicity of radiant heating elements, typically thermal generating lamps spatially arranged into a plurality of heating zones. In one construction, the heating elements are arranged to face one side of the wafer, while a plurality of sensing devices, such as infrared pyrometers or emissometers are arranged on the opposite side of the wafer to sense the temperature of the wafer backside.
Precise temperature control in RTP systems over the entire processing cycle is often critical to achieving acceptable processing results. This means that the temperature at each monitored location in the chamber must be maintained within certain limits during the temperature ramp-up and ramp-down sequences. When the temperature at one or more monitored locations is not maintained within desired limits, a fault occurs which may have a material adverse effect on the quality of the process, and thus on the quality of the processed wafer.
The task of accurately monitoring the temperature at multiple locations in the processing chamber is complicated by several factors. One factor affecting monitoring accuracy is related to the presence of foreign particles or residue within the chamber. Such foreign materials and residue are often generated in the processing of previous wafers or in residue or small broken fragments of wafers remaining in the chamber. Such residue and wafer fragments within the chamber alter the emissivity measurements taken by the temperature sensors, thus giving rise to inaccurate readings that may prevent the detection of temperature control faults. Inaccurate temperature readings can also result from the temperature sensors receiving radiant heat emanating from a source other than the wafer being monitored.
The construction of prior art wafer supports has contributed to both types of temperature measurement errors discussed above. Prior art wart wafer supports used in RTP systems were of a 2 piece construction, comprising a flat annular ring for supporting the peripheral edge of the wafer, and a downwardly extending cylindrical member supporting the ring and mounted for rotation within the processing chamber so as to rotate the wafer during the RTP process. It was not practical to achieve a complete seal between the 2 pieces on the wafer support, consequently a small space or gap was present between the 2 pieces. As a result, residue or small foreign particles present at the top of the chamber were allowed to pass through the gap in the wafer support, thus migrating into the lower part of the chamber, beneath the wafer, and settling on the temperature sensors thereby affecting temperature measurements. Additionally, radiant heat from the radiant heat source above the wafer was allowed to pass through the gap between the wafer support components or between the support and the wafer itself, thus passing into the lower half of the chamber. As a result, the temperature sensors received some amount of heat directly form the radiant heat source such that the heat sensed by these sensors did not originate entirely from the back side of the wafer which they were intended to monitor. Consequently, this direct reading of radiant heat emanating from the heat source resulted in inaccurate measurement of wafer temperature.
In addition to the deficiencies of prior art wafer supports mentioned above, the non-uniform surface and/homogeneity of the materials used to form the wafer support also contributed to temperature measurement error since non-uniform heating of the support prevented the wafer from achieving the desired temperature profile.
From the foregoing, it is apparent that there is a clear need in the art for an improved wafer support which eliminates the deficiencies of the prior art construction discussed above. The present invention is directed toward overcoming these deficiencies.
In accordance with the present invention, a semiconductor wafer support is provided for use in a thermally controlled process chamber provided with a radiant heat source and a radiant heat temperature sensor for sensing the temperature of the wafer. The support comprises a one piece member of unitary construction, having a horizontal section and a downwardly extending cylindrical section seamlessly interconnected by an elbow section. The horizontal section includes a radially inwardly extending annular surface forming a lip for supporting the periphery of the wafer thereon. In the preferred embodiment, the lip is formed in the horizontal section so as to define a recess to assist in registering and retaining the wafer in a desired position, while reducing the possibility of thermal radiation leakage between the wafer and the support. The horizontal section is mounted, as with magnetic means, for rotation within the chamber so as to rotate the wafer during the RTP process.
The wafer support if preferably formed of silicon carbide and is coated with a layer of silicon so as to be uniformly and highly absorbent of radiant heat.